Flat tube electrical device



Sept. 20, 1949.

E. LEMMERS FLAT TUBE ELECTRICAL DEVICE Filed Oct. 21, 1943- EugeneLemmers,

M His Ai? orneg.

Patented Sept. 2Q, 1949 FLAT TUBE ELECTRICAL DEV CE Eugene :Lemmers,Cleveland Heights, Ohio, as-

signor to General Electric Company, a corpora-'-- I tion of New York'Application fl ctober 21, l9 i3,'S'erial Korma This invention-relatestoelectrical devices having tubular envelopes; such as incandescent lampsand 'heating devices and electric discharge lamps and devicesof variouskinds, all of which generally have envelopes of vitreous material suchas glass or silica.

In thecase of "fluorescent lamps of the usual low-pressure positivecolumn discharge type in which phosphors'fare excited by the 2537 A.resonance radiation of mercury, .as well as in other resonance radiationlamps (such as germicidal mercury lamps and sodium lamps), it ispossible. to obtain' either: greatly improved efficicncy' of operationor much greater current capacity and discharge wattage when the lamptube. is oval or flatjf rather than substantially circular asinpresentpractice. Also, the light distribution from a "ilatitube lamp ismore favorable for somepurposes, than that from a round tube. But notwithstanding these and otheradvantages .of flat tubes .oyer round tubes,flat tubes as heretofore, made have presented a Very'serious drawbackwhich has prevented their use to any great extent: namely weaknessagainst collapse or implosionby external atmospheric pressurewhenthey,areevacuated, even if they contain an atmosphere at greatly reducedpressure, as is usual in ordinary fluorescent lamps and in some othervresonance radiationlamps, as

well as in tubular incandescent lamps. Because 0 of this, the only flattubelamp heretofore on the market has been a small, low-wattagefluorescent lamp known commerc'iallyas. the T2 /2 x 6, having aflattened or oval tube about inch wide by% inch thick, internally, andonly 6 to 9 inches long. .1 r

. I have founda way. ofovercoming this weakness of flat-tube lampsagainst external pressure, by putting the longitudinal edges. or cornersof the flat tube 2111.8, state of permanent precorn pression or bendingstrain, as I .term it. 'For maximum orlassuredsafe resistance-of thelamp against collapse or implosion, the strain in the tube edges may bemade so great that even after the lamp-is evacuated and the pressuretherein is brought to its minimum value during service of the device, adegree of such strain in the edges still remains, lover, and-abovethattaken up in compensating the external atmospheric pressure on thetube;,however,j'anu'improvement in the resistance to collapse'maybejproduced even'by lower degrees offprestr'ain .or' stress in the tubeedges,v The proper prestra'ining of the tube edges maybe broughtaboutuat any convenient 9 Claims. (01. 49-: 9) 1:

as an incident of drawing continuous flat tubing from molten glass;or,'at the-opposite extreme, aftera length of tubing has been fabricatedinto a "fluorescent lamp, bjut before-exhausting the lamp.

Without attempting'to define the possibly beneficial range oftubeflatness, itmaybe said that for the ordinary commercial range offluorescent lamp tube sizes from ljin'ch to 2 inches, in internaldiameter, flattening to a 4:1 ratio-of internal width tointernal-thickness is very decidedly advantageous, while flatteningmerely to a 2:1 ratio is'about th mallest degree of flattening that is of"any"real advantage. A strictly flat tube with semicylindricaledg'es orcorners is more advantageous than a pronouncedly oval one; but some"slight internal transverse concavity of the wide fflatf' -walls offersno material drawback. On the} other hand, appreciable transverseinternalfconvegity'of'the wide walls is decidedly to? vc ideg i.;fbecause even whel slight, it results'm'a tendencyof the discharge to oupy n y f. the ,QflYifiY. Q hthe t to jump back and forth ldtwee'nfftheregions at opposite sidesio'fthe lin f "'ini'n'imum internalthickness'of 'thecavity,. stea'd'iof constantly filling the wholecrossse ctionoffthe tube,'as is necessary if the advantage'of'a flattube is to be realized. Slight. internal transverse concavity of thewide walls offersafmargin of safety in manufacture against accidentalinternal convexity of these walls in particular instances.

The methodof putting the "edges or corners of aflat tube under stresswhichI at present prefer is by heat treatment: For thispurpose, the tubeis heated above the strain" poin ternperature of the vitreous material,andfthe longitudinal edges of the flattened tube: are, cooledor chilledbelow the strain point. before its wide and substantially orapproximately.flatlongitudirial walls cool below this temperature.Theihe'atingof the tube above itsstrain poir tas here referred to maybemerely incidental toidrawing outand'forming the glass into a flat tube,or may be a subsequentheating for therpurpose ofjflatten'in'g aninitially roundformed tube, or may be. 'aspecial heating for the mainpurpose ofimpo'singstrain or stressiat the corners or edges;The'prechilling oi the tube edges in advanceof tlqlejcoolirig of itswide walls may be efiected in a varietyiqi ways as by blowing coolingfluid (e "g' airj on the edges, wholly or partially to: the exclusion'of the wide walls; or by heating the widefwall's' to keep them abovethe strain point'while'the edges'are freely eX- stage of tube or lampmanufacture: for example, 55 posed to be'coole d "below this point bythe circumambient atmosphere or cooling fluid, and then stopping orreducing the heat to allow the wide Walls to cool; or even by merelycovering or lagging the wide walls to retain their heat, so that theatmosphere can cool the more exposed edges below the strain point beforethe wide walls cool down to or below this temperature. The strain poinhere referred to is the temperature below which vitreous material cannotbe permanently strained-a particular temperature which is a particularproperty of every kind of glass or other vitreous material, andgenerally. differs .for different materials-all as well understood inthe glass art.

It is, of course, importantthat when a-ilat tube has had its edges putin strain as described, it should never afterward be heated above thestrain point of the material in any subsequent step or operation of lampmanufacture; because this would relieve the strain and destroy theeffectual resistance of the tube to external pressure. The usualannealing of the tubeas an incident of lamp manufacture must bealtogetheromitted. 7 Various features and advantages of my inventionwill appear from the description of speciesor embodimentsv and modes ofexecution of the :invention as applied to fluorescent lamps, and fromthe drawings.

In the drawings, Figfl a perspective or tilted view of a form offlattubular -lamp in which my I invention may be applied fandembodied,portions of the envelope wall being broken out toexpose parts within andto" shorten thef'figure; and Fig. 2 is a cross-section of theltub'takenas indicated by the line and 'arrows 2-i-2 in Fig.1, and on a largerscale. f

Fig. 3 is a side view of apparatus for flattening and straining alam'p'tubeinf'acordance with my invention, a portion of thetube beingbroken out to shorten the figure; and F g; 4"shows a crossa sectionthrough the apparatusandthe lamp tube, taken as indicated by thel'in andarrows 4-4' in Fig.3. g

Fig. 5 is a perspective 61 tiltedvi'ew of another form of flat tubularlani'p 'in'which my invention may be embodied; and Fig;- 6 shows alongitudinal section through oneend of the lamp tube shown in Fig. 5, ona larger scale."

Fig. 1 shows a fluorescent" lamp L of ordinary low pressure positivecolumn dischargeltype in I;

which my invention may be embodied. As here illustrated, the circularlyround tube ends I0, If! are annularly reduced or; shouldered and aresecured in bases l2, l2 eachprovided with bipost contact terminals I3,I3. 7 Asshown at one end of the lamp L, the cathode mount Mlc'omprises acathode M of coiled-coil tungsten wire filament type, which may beactivated 'with'a coating of alkaline-earth-metal oxidels); suchas theusual mixture comprising barium and strontium oxides.

The current inlead wires ,15, I5 connecting the ends of the cathodecoils I4 to-thefcorresponding biposts |3, |3 are sealed through the stemflares 58 that form the end walls of the tubular lamp envelope, one ofwhich is s hown provided with an exhaust tube sealed or tipped off atl8. The leads l5, l5 are shown as provided with anode, extensions |9,IS. The lamp tube L'may contain a low pressure starting atmosphere ofone or more of the inert rare group O gaseasuch as argonat a pressure ofsome 2 to 5mm. of mercury. A supply of ionizable and vaporizable, metalto serve as a working substance and provide the discharge atmosphere inoperation is also indicated at 20 as a droplet of mercury exceedinginamount what 4 will be vaporized during the operation of the device. Acoating of luminescent material or phosphor is indicated on the insideof the envelope wall at 2 I, to be excited by the mercury discharge, andespecially by its resonance radiation of 2537 A. wavelength.

As thus far described, the lamp L is essentially similar to ordinaryround tube fluorescent lamps now in common 1159.

Unlike ordinary fluorescent lamps, the lamp tube L is here shown asflattened substantially throughout its length-between the round ends m,I!) that carry the bases |2, |2 to an elongated loop-like cross section,Fig. 2, comprising wide, flat walls 22, 22 interconnected by thesemicircularly rounded envelope edges 23, 23. The round ends HI, ID maymerge into the flattened portion with a wedge-like taper indicated at24, 24, Fig. 1. It is the rounded edges 23, 23 that are brought into acondition of surface compression in the material in order to render thetube resistant to the external atmospheric pressure.

Figs. 3 and 4. illustrate a form of treating apparatus F which maybeused for straining'the edges 23, 23 of a flat lamp tube.- As shown inFig. 3, the apparatus comprises counterpart long, flat metal bars 30,,(as of iron) which are oppositely and slopingly ofiset at 3|, 3| neartheir ends, in correspondence to the wedge-like taper of the tube L at24, 24 in Fig. 1, so that their end portions 32, 32 are further apartthan the intermediate portions of thebars. These bars 30,33 are madesubstantially narrower thanthe width of the tube L.. Provision is madefor maintaining the bars 30, 30 substantially parallel and guiding theirrelative sidewise movement, as by means of guide rods orpins 33,, 33fixed in the end portions 32, 32 of a bar and extending throughcorresponding' holes in the portions .32, 32 of the other. bar; and thusthe bars are'kept in facial registry or alignment directly. opposite oneanother. Each bar 30 is. faced with heat insulating material or laggingto reduce and-slow up the transmission of heat from a tube'L between thebars to them, as by wrapping asbestos cloth tape 35 .around the bar 30in overlapping convolutions. Preferably this insulation may extendbeyond the bar ofisets 3|, 3|, and may nearly cover the bar end portions32, 32. With each bar 30 may also be. associated suitableelectricalheating means3l, here shown as a commercial form of stripheater comprising a suitably insulated resistance unit] encased in aflattened tubular sheet metal sheath; with terminal connectors orbinding posts 38, 38 at each end of the heater31. Each heater3'lliesalong the back side of its bar 30, substantially through- -out itslength between the offsets 3|, 3|, and is suitably held thereagainst, asby wrapping the asbestos sheathing 35 around the heater and the bartogether. v

In using'the apparatus F, provision is made to exert a yielding force onthe bars 30, 30 to press them toward one another; which may be done byarranging them one directly above the other as shown in Fig. 3, thustaking full advantage of the weight of the upper bar. The bars 30, 30having been heated up fully by their heaters 31, 31, and an alreadyflattened lamp tube L having been heated well above the strain-pointtemp rature of itsvitreous material in .any suitable way-as in a heatingfurnace or lehr, not shown,the heated tube L is placed between the bars.30, 30 asshown in Fig. 3, with the tube resting on the lowerbar and thetop bar resting on the tube so that the full force of its weight(including that of its gisaiei .5 heater 31) is exerted on the tube.This weight is so chosen that the tube L is strained or even furtherflattened slightly before the exposed edges 23, 23 have cooledoff to thestrain-point, below.

which they can no longer be further permanently bent or-flattened by theload. While this is taking place, the coolingrof the wide,:flattenedtube walls 22, 22 is limited and retarded by the asbestos facings 35,35, and also by heat from continued operation of the heaters 31, 31, ifnecessary, so that the walls 22, 22 do not cool down to and below thestrain-point ahead of the edges 23, 23. Once the edges '23, 23 are belowthe strain-point, however, the heaters 31, 31 may be cut outof'circuit'so that the walls 22, 22 may cool off below the strain point;and as soon as convenient after this, the tube L may be removed from theapparatus F.

As shown in Fig. 4, thewidewalls 22, 22 are very slightly convex outwardand concave inward, instead of being perfectly flat as in Fig. 2; butthis concavity is so slight as not to impair the advantages of the flatshape appreciably.

Fig. 5 shows a lamp whose envelope tube L is flat right out of its veryends Illa, Illa, which are closed by substantially flat end wall discsIlia, |6a instead of by stem-flares l6, l6 as in Fig. 1. In otherrespects, the cathode mounts Ma, Ma are just like the cathode mounts Min Fig. 1, and so need not be further described. As shown in Figs. 5 and6, the end discs 16a, l6a are of sheet metal such as chrome iron, andthe tube ends Ilia, Illa abut directly against their margins and aresealed to them by fusion of the glass. The extreme edge of each discI60. is shown in Fig. 6 as turned or beaded over at 40 toward the tubeend Illa around the seal, thus protecting the latter and minimizing thepossibility of the disc being pulled away so as to impair the seal, bysomething catching behind the edge, besides stiffening and reinforcingthe disc. is further strengthened by a central inward -circularembossment 4| between the current inlead wires I5a, I541. One of theseembossments 4| is apertured through its bottom in correspondence withthe end of an exhaust tube fused and sealed to the bottom, and alsoshelters and protects the exhaust seal tip [8a formed by sealing off theexhaust tube after the lamp L has been exhausted and charged withstartin gas and working substance.

Instead of being of usual long bipost type, the contact terminals ofeach end disc lBa shown in Figs. 5 and 6 are in the form of pinscomprising fiat or slightly rounded heads I311, I311 attached (as bywelding) to the outer ends of the current leads I5a, I5a, which form thepin shanks, and projecting or outstanding from the outer disc face. Forthis purpose, insulative spacing washers 42, 42 are interposed betweenthe pin heads 13a, 13a, and the disc lBa; they may consist of glasssealed by fusion to the outer disc surface and into the disc holesthrough which the shanks i5a, 15a extend, as well as to the rearsurfaces of the heads l3a, l3a, which may be of chrome iron.

The tube and end construction illustrated in Figs. 5 and 6 lend itselfespecially to the use of tubing produced directly from the glass batchin flat form, with its edges 23, 23 properly strained. The weakening ofthe edge strain at the tube ends Isa, Illa in sealing them to the enddiscs lea, Ilia by fusion is amply compensated by the strength againstcollapse imparted to the tube ends by their strong attachment to thediscs.

Each disc IBa .6 In Figs. 3 6, various parts andfeatures are marked withthe same reference characters as "their counterparts in Figs. 1 and 2,as a means of dispensing with repetitive description, a distinctiveletter being added where this seems needful. However, I make no claim tothe special end-disc and contact terminal features illustrated in Figs.5 and 6, since they are the invention of Paul O. caries, and are claimedin his application Serial No. 506,230, filed October 14', 1943, whichissued as Patent No. 2,415,367, February 18, 1947. V What I claim as newand desire to secure by Letters Patent of the United States is:

1; An envelope for evacuated electric devices comprising a flattenedtube of vitreous material having only its longitudinal edges in a stateof surface compressionsuflicient in amount to effecti'vely compensatestresses set up by atmospheric pressure and tending to rupture the tubewhen it is evacuated.

-2. An envelope for evacuatedelectric devices comprising a flattenedtube of vitreous material having rounded longitudinal edges andsubstantially flat wide walls with only its longitudinal edges in astate of surface compression sufficient in amount to effectivelycompensate stresses set up by atmospheric pressure and tending torupture the tube when it is evacuated.

3. In the manufacture of a flattened envelope tube of vitreous materialhaving narrow longitudinal edges and wide longitudinal walls, the methodof rendering the tube effectively resistant to collapse by externalatmospheric pressure when the tube is evacuated, which comprisessubjecting the said longitudinal edges and wide longitudinal walls todifferent thermal treatments which will produce a state of surfacecompression in only the said longitudinal edges.

4. In the manufacture of a flattened envelope tube of vitreous materialhaving narrow longitudinal edges and wide longitudinal walls, the methodof rendering the tube effectively resistant to collapse by externalatmospheric pressure when the tube is evacuated, which comprises heatingthe tube substantially uniformly over its whole surface to a temperaturewell above the strain point of the vitreous material and cooling thetube more strongly at said longitudinal edges.

5. In the manufacture of a flattened vitreous envelope tube havingnarrow longitudinal edges and wide longitudinal walls, the method ofrendering the tube effectively resistant to collapse by externalatmospheric pressure, when the tube is evacuated, which method compriseschilling the longitudinal tube edges from a temperature above the strainpoint of the material to a temperature below the strain point in advanceof the cooling of its wide longitudinal walls below the strain point.

6. In the manufacture of a flattened vitreous envelope tube havingnarrow longitudinal edges and wide longitudinal walls, the method ofrendering-therwtube effectively resistant to collapse by externalatmospheric pressure, when the tube is evacuated, which method comprisesheating. the tube above the strain point of the material, cooling itslongitudinal edges below said strain point while maintaining itsflattened longitudinal walls above the strain point, and thereafterallowing said walls to cool off, as well as said edges.

8. An electric lamp comprising an evacuated elongated envelope ofvitreous material, at least a portionof said envelope being flattenedand having narrow longitudinal edges and wide longitudinal walls, thesaid longitudinal edges only of the said envelope being in a state ofpermanent surface compression.

9; An electric lamp'comprising an evacuated elongated envelope ofvitreous material flattened in cross-section throughout its length, ametallic closure member fused to and closing each end of said envelope,the longitudinal edges only of said envelope being in a state ofpermanent surface compression.

. EUGENE LEMMERS.

eb h P tent UNITED: STATES PATENTS Number Name Date 1,016,320 BurnsideFeb. 6, 1912 1,483,461 Littlton, Jr. Feb. 12, 1924 1,981,560 Littleton 1Nov. 20, 1934 1,994,312 1 Herre Mar. 12, 1935 2,001,852 Burner 1 May 21,1935 2,055,981 Magriifi Sept. 29, 1936 2,062,836 Scott 1 Dec. 1, 19362,244,715 L0Iig June 10, 1941 V FQREIGNv PATENTS Number" f I CountryDate 486,325 Great Britain June 2, 1938

